This application claims benefit of and priority from: European Applications No. 98202377.2 filed Jul. 8, 1998 and No. 98870255.1 filed Nov. 19, 1998; International Application No. PCT/BE99/00088 filed Jul. 7, 1999; and U.S. application Ser. No. 09/743,287 filed Jan. 8, 2001, the contents of all of which are incorporated hereinto by this reference.
The present invention relates to a flat product, such as a sheet, made of steel containing Mn and having multiple phases with a ferrite matrix.
High-strength steels such as rephosphorized steels, microalloy steels and bake-hardening steels are widely used in automobile components. Sheet made of such a steel requires a strength sufficient to meet automobile safety and must, in addition, have excellent forming properties.
It is also known that the strength and ductility of a multiphase steel may be improved by a multiphase microstructure possibly combined with transformation-induced plasticity (TRIP) by transformation of residual austenite into martensite.
The TRIP effect was first discovered by Zackay et al. in steels containing large amounts of nickel and chromium.
However, the presence in these steels of large amounts of such alloying elements poses problems when manufacturing steels under economically profitable conditions.
It should also be pointed out that a steel containing a significant amount of residual austenite may be obtained by the addition of silicon and of manganese and by hot rolling with a controlled heat cycle or by cold rolling followed by intercritical annealing combined with a bainitic soak producing a structure formed from several phases and with isolated regions or islands of residual austenite, this structure being stable at room temperature.
Such a steel is known in particular from document U.S. Pat. No. 4,544,422 and from the articles by Gregory H. Haidemonopoulos xe2x80x9cAustenite stabilisation from direct cementite conversion in low-alloy steelsxe2x80x9d in Steel Research 67 (1996) No. 3, pp 93-99 and xe2x80x9cModelling of austenite stability in low-alloy triple-phase steelsxe2x80x9d, Steel Research 67 (1996) No. 11, pp 513-519.
However, these known steels and their smelting have a number of not-insignificant drawbacks since, in general, they require large amounts of addition elements.
In addition, the fact that in these known steels the silicon content is generally relatively high, about 1.25 to 1.50% of Si, gives a steel sheet having certain surface defects called xe2x80x9ccat""s tonguexe2x80x9d defects which are created during the hot rolling. Moreover, these known steels pose problems when dip-coating them with metal, such as during galvanizing, due to the embrittlement of the coating and to problems of wettability of the liquid metal intended to form the coating.
One of the essential objectives of the present invention is to provide a steel sheet which makes it possible to remedy the abovementioned drawbacks and which, thus, is particularly well suited for galvanizing while still being able to be manufactured according to an economically very favorable process.
According to the invention, the steel has improved strength and ductility properties while still having an extremely low silicon content, so that it is very particularly suitable for being formed and for being surface-treated in the automobile industry.
For this purpose, the steel sheet according to the invention has a structure and properties which can be obtained by a process as defined in claim 1.
Advantageously, the steel according to the invention has the following chemical composition: 0.05 to 0.8% C; 0.2 to 3.0% Mn; Sixe2x89xa61.0%; Bxe2x89xa60.100%; Ti, Nb, Zr and V eachxe2x89xa60.200%; Alxe2x89xa60.400%; Nxe2x89xa60.100%; Pxe2x89xa60.100%; Cr, Ni and Cu eachxe2x89xa62.000%; Moxe2x89xa60.500%.
The invention also relates to the particular aforementioned process for the manufacture of the flat steel product according to the invention.
According to one particular method of implementing this process, the aforementioned tempering is carried out at a temperature below the eutectoid temperature (A1) so as to form cementite and to make carbide-forming elements, such as Mn, to diffuse into this cementite.
According to a preferred method of implementation, the process according to the invention comprises a step of cooling down to room temperature after the aforementioned soak or tempering step, this step then being followed by a cold-rolling step before the aforementioned heat-treatment step.
Further details and features of the invention will become apparent from the description given below, by way of nonlimiting example, of a few particular embodiments of the invention with reference to the appended drawings.